Heating-induced negative temperature coefficient effect in conductive graphene/polymer ternary nanocomposites with a segregated and double-percolated structure

Electrically conductive polymer composites (CPCs) show considerable promise in thermistors owing to their characteristics of positive temperature coefficient (PTC) effect and negative temperature coefficient (NTC) effect of resistance. In contrast to traditional rigid ceramic thermistors, CPCs are lightweight with good processibility, flexibility and variety. However, the development of polymer-based NTC thermistors has been impeded by the polymer volume expansion effect, which usually leads to a PTC effect. Here, we employed a segregated and double-percolated composite microstructure to inhibit the polymer volume expansion effect and flake-like graphene as conductive filler to construct a resistant conductive network by its overlapping contact mode, targeted at developing a favorable NTC material. This strategy was carried out by selectively distributing graphene in a polyamide 6 (PA6) phase between isolated ultrahigh molecular weight polyethylene (UHMWPE) particles. As a result, the graphene/PA6/UHMWPE composites exhibited a relatively linear NTC effect through the whole heating process, a high NTC intensity of 5.1, a wide temperature range of 30-260 degrees C, good reproducibility as well as high mechanical properties. The underlying mechanism of the NTC effect originates from the morphology evolution from crumpled to stretched morphology, enhanced electron mobility in the crumpled morphology, and the improved conductivity of graphene triggered by increasing the temperature.